1. Field of the Invention
This invention relates to a disc recording/reproducing apparatus, and more particularly to an eccentricity suppressing servo method and apparatus that is adapted to effectively suppress an eccentricity occurring when a disc is driven.
2. Description of the Prior Art
Nowadays, an optical recording medium has been prevalent as a recording medium for recording a high density information. The optical recording medium includes compact disc(CD) series discs and digital versatile disc(DVD) series discs having a recording density much higher than the CD series disc, etc. In such a disc type recording medium, an eccentricity occurs when the recording medium is driven. This is caused by an error generated at the manufacturing process of the disc and an error existing in a disc mechanism(i.e., a deck) included in the disc driver. Due to the eccentricity of disc, a laser light beam fails to be trace accurately along signal tracks on the optical disc. In other words, a tracking control can not be performed accurately at the disc driver. Due to this, the disc driver fails to record and reproduce the optical disc accurately.
In order to suppress the eccentricity of disc and perform the recording and reproduction of optical disc accurately, the optical disc recording/reproducing apparatus employs an eccentricity suppressing servo apparatus of feedback control system wherein a loop gain of the control system is increased within a range in which the control system is kept stabbly. As shown in FIG. 1, the eccentricity suppressing servo apparatus of feedback control system includes an adder 2 connected to make a single loop, a gain controller 4, and a planter 6. The adder 2 receives a feedback signal Y(t) from the planter 6. This feedback signal Y(t) includes an error component caused by an eccentricity of optical disc and error component caused by the outside interference as a tracking error signal. The adder 2 adds a reference signal Ref from an input line 1 to the feedback signal Y(t) to calculate a tracking correction amount. The gain controller 4 multiplies the tracking correction amount calculated with the adder 2 by a predetermined value to generate a tracking control signal having an increasing gain. In other words, the gain controller 4 responds to the tracking correction amount to increase a loop gain. Also, the gain controller 4 limits a gain of the tracking control signal in such a manner to exceed a certain value, thereby stabilizing the feedback control. The planter 6 responds to the tracking control signal from the gain controller 4 to move the position of a light beam in the width direction of signal track. Also, the planter 6 detects a degree at which a light beam departs from the center line of signal track, that is, a tracking error signal from the distribution of a light beam reflected by the optical disc, and applies the tracking error signal to the adder 2 as the feedback signal Y(t). In order to perform such a function, the planter 6 includes an optical pickup that has an actuator(not shown) for driving an objective lens and a photo detector(not shown) for detecting a reflective light from the optical disc, etc. In addition, the planter 6 includes a detection circuit for detecting the tracking error signal from a signal detected by means of the photo detector.
In The eccentricity suppressing servo apparatus of feedback control system as described above, an affect caused by the eccentricity of disc can be suppressed by increasing a loop gain depending upon the feedback tracking error signal, but a response speed becomes slow with respect to a disc eccentricity amount above a certain limit and the disc eccentricity amount above a certain limit can not be suppressed appropriately because a loop gain is limited within a certain limit, that is, within a range in which a control system is kept stabbly. Due to this, in a disc driver employing the feedback control system eccentricity suppressing servo apparatus as described above, a light beam fails to be trace along the signal track and, furthermore, it is difficult to perform a recording/reproducing operation of an optical disc accurately. An affect caused by such a disc eccentricity becomes more serious as a recording density of optical disc becomes higher, that is, as a distance between the signal tracks becomes shorter. Further, in a DVD-RAM including recording fields having land or groove signal tracks LS or GS and header fields having embossed pit trains, an affect stemming from the disc eccentricity becomes much more serious.